1. Field of the Invention
The invention relates generally to the field of subsurface exploration and production. More particularly, the invention relates to methods and apparatus for measuring resistivity properties of earth formations penetrated by a wellbore.
2. Background Art
Resistivity logging tools have been used for many years to measure the resistivities of earth formations surrounding a borehole. Traditionally, resistivity measurements were obtained by lowering a wireline-conveyed logging device into a wellbore after the wellbore was drilled. However, the wireline measurements necessarily involve a delay between the time a well is drilled and when the measurements are acquired. A preferred approach is to make such measurements while the well is being drilled so that corrective steps may be taken if necessary. For example, wellbore information if available in real time may be used to make adjustments to mud weights to prevent formation damage and to improve well stability. In addition, real time formation log data may be used to direct a drill bit to the desired direction (i.e., geosteering). On the other hand, if the measurements are taken after a delay, drilling fluids (“mud”) may have invaded the formation and altered the properties of the near wellbore regions. For these reasons, logging-while-drilling (LWD) and measurement-while-drilling (MWD) techniques have been developed. LWD will be used to include both LWD and MWD techniques in this disclosure.
FIG. 1A illustrates a typical LWD system disposed in a wellbore. A drill string 1 is suspended within a borehole 3 with a drill bit 5 attached at the lower end. The drill string 1 and attached drill bit 5 are rotated by a rotating table 9 while being lowered into the well. This causes the drill bit 5 to penetrate the formation 11. As the drill bit 5 penetrates the formation 11, the mud is pumped down through a central bore of the drill string 1 to lubricate the drill bit 5 and to carry cuttings from bottom hole to the surface via the borehole 3 and mud flow line 13. Located behind drill bit 5 are sections of LWD drill collars 15, which may include an array of resistivity sensors 15a or any other type of sensor known in the art. It will be understood that “sensors”, as used in this disclosure, includes antennas, toroids, and electrodes (which may be operated as transmitters and/or receivers). The resistivity sensors 15a measure the resistivity of the formation 11 as the formation 11 is penetrated by the drill bit 5, acquiring the measurements before the mud invades the formation 11.
In general, there are two types of LWD tools for measuring formation resistivity—lateral and induction or propagation tools. Each of these tools relies on an electromagnetic (EM) measurement principle. Propagation-type tools emit high-frequency electric fields into the formation to determine borehole and formation responses by measuring voltages induced in the receivers or by measuring difference responses between a pair of receivers or between the transmitter and the receiver. For example, for a propagation tool, incoming signal phases and amplitudes may be measured at each of several receivers with respect to the phases and amplitudes of the signals used to drive the transmitter. Induction-type transmitters generate magnetic fields that induce currents to flow in the formations. These currents generate secondary magnetic fields that are measured as induced voltages in receiver antennas disposed at a distance from the transmitter antenna. Induction and propagation tools work best in wells drilled in relatively conductive formations using relatively non-conductive muds, including insulating muds (e.g., oil-based muds). Typical induction and propagation tools are not configured to resolve resistivity variations around the wellbore.
Conventional induction or propagation tools use wound coils or solenoids as transmitter and receiver antennas. The antennas are disposed on the instrument by winding a coil around the tool body, encapsulating it in an insulating filler and then sealing the entire assembly with rubber. Although induction tools and propagation tools are generally operated at different frequencies, and in some instances used to probe different subsurface properties (e.g., detecting formation dielectric properties with propagation tools), in most instances they are used in a similar manner to measure formation resistivity. Thus any reference to induction herein is understood to be interchangeable with propagation, and vice-versa.
A lateral tool typically uses one or more antennas or electrodes to inject low-frequency transverse magnetic fields into the formations to determine borehole and formation responses by measuring the current flow through the formations to the receivers. This technique works best in relatively resistive formations drilled with conductive muds, such as water-based muds. Lateral resistivity tools are generally responsive to azimuthal variations in formation resistivities around the borehole.
To transmit a transverse magnetic field into a formation, a lateral tool typically uses a toroidal transmitter, which is built by wrapping a conductive wire around a donut-shaped, magnetically permeable core (a toroidal core). To detect currents that flow in the formation, a lateral tool uses an electrode (e.g. ring electrode or button electrode) receiver or a toroidal receiver. In conventional LWD tools, the toroidal transmitter or receiver is typically built in a sleeve that is slipped onto the drill collar at the final stage of assembly.
FIG. 1B illustrates a typical lateral resistivity tool. As shown, the tool includes two transmitters T1, T2 disposed on a drill collar 15. Two monitor antennas M0 and M2 are also included. The transmitter (current injector) antennas T1, T2 and the monitor antennas M0, M2 are shown as toroidal coils, which will be described in detail below. The resistivity tool may also include other electrode receivers, such as a ring electrode R and button electrodes B, B′. The ring electrode R and the button electrodes B and B′ are conductive electrodes disposed on the collar 15, but they are electrically isolated from the collar 15 by insulating materials. A ring electrode R is a conductive metal band disposed around the circumference of the collar 15. The ring electrode R typically measures an azimuthally averaged current. On the other hand, button electrodes B and B′ are typically disposed on one side of the tool. The button electrodes B and B′ are capable of azimuthal measurements and high-resolution imaging.
As noted above, the induction/propagation sensor works best in relatively low resistivity (or conductive) formations drilled with resistive muds, including oil-based muds. However, such tools are typically not configured to resolve resistivity variations with azimuthal sensitivity around the wellbore. Lateral tools are more suitable for resistive formations drilled with conductive muds, and lateral measurements using button electrodes are generally sensitive to azimuthal variations.
Because the lateral and induction/propagation devices work particularly well in certain environments, they compliment each other. However, a driller may lack the necessary information to make a proper choice regarding the type of tool(s) to use for a particular well. Therefore, different types of logging tools are often used together in a single logging run. In wireline operations, a lateral toot is often run with an induction tool in the same run to provide a shallow depth of investigation and to provide better identification of zones invaded with conductive mud. It is not operationally efficient, nor cost effective, to run these tools on separate passes into the well. In addition, separate logging passes can introduce inaccuracy when trying to determine pre-invasion formation resistivity. Inaccuracy is also introduced because the measurement signal path, with respect to the formation interval and geometry, changes from one logging pass to the next. Therefore, providing different types of sources/sensors in one tool or system for multi-mode resistivity measurements is desirable.
An example of resistivity logging using two types of sensors in a single tool is disclosed in U.S. Pat. No. 5,428,293 issued to Sinclair et al. The logging methods described in this patent use low and high frequency sensors to provide measurements at multiple depths of investigation to monitor mud invasion. Although these methods propose to use a tool having both low and high frequency sensors in the same drill collar, no detail is given as to the construction of the tool.
In designing any sensors for use in an LWD tool, shields that can withstand the abrasive and harsh environments during a drilling operation are essential. Because the lateral and propagation resistivity sensors operate under different EM measurement principles, they have different shield requirements. LWD tools having propagation resistivity antennas built into recesses in the collar wall and fitted with protective shields are known in the art. Propagation tool configurations are further described in U.S. Pat. No. 5,594,343 issued to Clark et al.
FIG. 2A shows a cross-section of a typical drill collar 21 equipped for a propagation resistivity measurement. The collar 21 includes a recess 29 formed circumferentially around the collar exterior to some desired depth. A propagation resistivity sensor 25 is disposed in the recess 29. The collar 21 is equipped with an inner sleeve or chassis 26 disposed therein to form a void to house an electronics module 22.
The module 22 is coupled to the sensor 25 via an electrical connection 27 traversing a feedthrough 28 within the drill collar 21 wall. The sensor 25 is potted within the recess 29 (e.g. with fiberglass filling 20) and covered with a rubber overmolding 19. A shield 23 is attached atop the overmolding 19 over the recess 29 to protect the sensor 25 from damage during the drilling process. The collar 21 may also be fitted with a wear band 38 for added sensor protection. As shown in FIG. 2B, the shield 23 includes a plurality of longitudinal slots 24 filled with an insulating material as known in the art.
A lateral resistivity sensor (e.g., a toroidal antenna) induces a magnetic field in the formation. FIG. 3A shows a conventional lateral resistivity sensor that is disclosed in Bonner et al., “A New Generation of Electrode Resistivity Measurements for Formation Evaluation While Drilling,” SPWLA, 35th Annual Logging Symposium, Jun. 19-22, 1994, Paper OO, and U.S. Pat. No. 5,339,037 issued to Bonner et al. An LWD collar 31 is shown. A lateral resistivity sensor is constructed as a sleeve 30 that is slipped over the drill collar 31 and fastened in place.
FIG. 3B shows an enlarged portion of the lateral sensor 30 described in the Bonner et al. patent. As shown, a toroidal antenna 35, including a conductive wire 33 wound around a core, is embedded in an insulating material 36 and protected by a metal shield 37. In order to permit a transverse magnetic field to be induced in the formation, the shield for a lateral sensor should not short circuit the current. Only one end, the upper end, of the conductive shield 37 contacts the drill collar 31. U.S. Pat. No. 3,408,561, issued to Redwine et al., describes toroidal antennas having metal protective outer walls. The proposed toroidal antennas are constructed in metal cylinders that are slipped over and screwed onto a drill collar.
There exists a need for downhole tools that provide for the combined acquisition of resistivity measurements using both lateral and propagation/induction types of resistivity sensors. It is also desirable that such tools have the sources/sensors directly integrated on the instrument.